1. Field of the Invention
This invention relates generally to comparator circuits and, more particularly, to a circuit having an output which switches between first and second states when an input voltage (or current) reaches a controlled trip point having a controllable positive, zero or negative temperature coefficient.
2. Description of the Prior Art
It is often necessary to monitor an input voltage (or current) to determine if and when it reaches a predetermined value. For example, the output voltage of an automotive battery is monitored in order to determine when the alternator must be turned on. That is, if the battery voltage is too low, the alternator must be turned on to recharge the battery. When the battery voltage exceeds a predetermined value, the alternator must be turned off. Prior art arrangements which perform the required monitoring function are generally inflexible with respect to both the location (amplitude) of the trip point and the amount and sense of its temperature coefficient.
It may be necessary to monitor an input voltage (or current) to determine when two or more trip points have been reached. Again using an automotive battery as an example, it may be necessary to generate a low voltage warning at a first trip point, to regulate the battery output at a second higher voltage, and to generate a high voltage warning at a still higher voltage. A prior art approach would be to construct a voltage reference circuit circuit from which a plurality of trip points are derived which are in turn applied to a plurality of traditional comparators which are monitoring the input voltage. This approach is not only cumbersome, but also renders the assignment of specific temperature coefficients to the trip points rather difficult.
One known circuit which provides for the generation of a temperature coefficient which may be varied from dV.sub.BE /dt (a negative number) to some positive value is described in U.S. Pat. No. 3,887,863. This circuit offers some flexiblility with respect to the positive temperature coefficient, but not with respect to the negative temperature coefficient.